Piezo-electric oscillator



Dec'. 18, 1934. w, M. KNOTT PIEZO ELECTRIC OSCILLATOR' Filed Sept. 2, 1931 OSCILLATOR fr T ATTORNEY Patented Dec. 18, 1934 UNITED STATES PATENT o-i-Fics r1,984,533 y Plazo-ELECTRIC oscILLA'roB William M. Knott, Maplewood, N. `J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application September Z, 1931, Serial No. 560,740'

7 Claims. (Cl. 171`-327-`) This invention relates to a piezoelectric crystal vibrating element and has for its object the physical control of such an element in its holder in such a manner that it will begin to vibrate more readily when used in an oscillating circuit,

and will control more power, than those formerly used.

A feature of the invention is the use of two silk threads or the like arranged to separate a piezoelectric crystal element from its electrodes and to form a cushion: for it.

In a specific radio transmittor of 10 watts power output which has. been designed in the laboratories of` applicants assignee for use at airports for short distance communication with aircraft, two specific problems were encountered. Onewas due to the desire'to design a transmitter which could bel built at minimum cost for this power output. This required that the crystal itself.` shouldvibrate withy suflicient amplitude to control considerable power, and thereby reduce the number of stages of amplification required. The other problem was due to the frequency at which the crystal was designed to vibrate. The frequencies`y to be used inthe service for which this transmitter is designed are from 230 to 500 kilocycles. was cut with suiiicient thickness to'enableit t0 vibrate at these frequencies, there was difficulty in making it start to-vibrate immediately when potential was applied to theoscillator. Exhaustive experimentation led to the discovery that if two threads, preferably of silk, were. tied loosely around the crystal or otherwise were arranged so that they formed acushion between the crystal and its electrodes, there-was no longer any diiculty about starting, and the power output of the crystalwas unusually large. In a life test a crystal was caused to oscillate over 1,500 hours, after which the threads were examined for deterioration. So far as could be determined, the threads were in exactly the same condition as at the start of the test. To determine whether the crystal can be depended on to start vibrating every time the plate voltage is applied to the vacuum tube, a test was made by applying the plate voltage over 10,000 times. The inventionwas found to be perfectly satisfactory in this respect.

The invention will be more fully understood from the following specification taken with the drawing on which like reference numerals designate elements of similar function and on which:

Fig. 1 is a cross-sectional view of a crystal holder with a crystal in position, tied loosely It. was-found. that when a crystal with threadsv according to this invention, and connected in a circuit to form a crystal con'- trolled oscillation generator. and

Fig. 1A isi-a. large view of the crystal and associated threads, which are illustrated in Fig. 1.

Fig. 2 is a partial cross-sectional view of a holder similar tothat illustrated in Fig. l and containingI acrystal, fabric being interposed between the crystal and the electrodes.

Fig. 3 is another partial cross-sectional view of a holder similar to that illustrated in Fig. l and containing a crystal, paper being interposed between the crystaland the electrodes.

The crystal holder shownin Fig. l is identical in many respects withV the crystal holder comprising an inverted cup-shapedy bottom electrode, a temperature responsive` element embedded in Woods metal in the cup and a heating element wound around the cup, which holder is disclosed andI claimed in Patent 1,921,432, granted August 8, 1933 to Burrell Stallard and the yholder havingy thev same design, except that the heating element is embedded in the Woods metal, which is disclosed and claimed in Patent 1,921,433', granted August 8, 1933 to Ole M. Hovgaard, both patents' being assignedv to applicants assignee.

In Fig. l, a cup-shaped base or mounting member 1, preferably o'finsulating material, is provided with a cover 2, which is secured to the base by bolts 3.l Withink said base is an invertedy cup 4 which1serves 'as the lower electrode for a piezoelectric; crystal 5, preferably of quartz material. An upper electrode 6 is above the crystal. Tied loosely around the crystal are two silk threads 29 and 30. Attached to the upper surface of the-upper electrodeis a metallic conductor 7 which maybe a thin iiexible strip of copper. This is attached to a hollow cylindrical metallic member 8 which may serve as a binding post for connection of the upper electrode to the grid of the associated tube. Within the cylindrical member 8 and coaxial therewith is a pin 9 surrounded by a spring 10. A wire 1l may be fastened to hooks 12 and 13 which are fixed to the cover 2, through grooves 14 and 15 in the cylindrical member to compress the spring and put pressure on the upper electrode by the pin, so that the electrode and crystal will not shake about during shipment of the apparatus. Projections 16 and 17 on the pin 9 prevent the pin and spring from coming out of the apparatus after the wire 11 is removed. When the apparatus is put in condition for use, the wire is cut so that the pin may rise and release the pressure on the upper electrode.

The cup 4 contains a thermostat 18 and a heating coil 19 embedded in a material having a low melting point and good heat conducting properties, such as Woods metal. The assembly of these elements is accomplished as follows: The cup is placed upright and the thermostat and heating coil are then placed in position, the thermostat protruding through an aperture in the wall of the cup. The aperture surrounding the thermostat is then filled with plastic4 material such as plaster of Paris, for example, which is allowed to harden, and the -cup is filled with molten Woods metal whichY is Aallowed to cool and harden. The cup isv then placed in position in the base, the thermostat.

projecting through an aperture therein. This aperture in the base is then iilledV with plastic material, which may be the same kind 'of material as was used before, and the base is ready for assembly with the rest of the structure. A metallic guard 20 surrounds Vthe exposed end of the thermostat and protects it from injury. A contact element 21 in the bottom of the thermostat'makes contact with one end of the heating coil'. .This end ofthev heating coil is electrically connected with a prong 22project ing through the basemember. :The thermostat also has a Contact element 23 which is connected by means of a rlead 24 to a second prong25 similarly projecting through the base member. These prongs are provided-to facilitate replacement of the holder in case a change of frequency is desired or in case of a breakdown. The lower thermostat contact, the end of the heating coil to which' it is connected, and thelower electrode are at a common ground potential so that suitable connections may be made kto electrical apparatus to be yassociated with the device to complete separate circuits `through the heating coil, thermostat, and across the electrodes. A third prong 26 may be providedto'complete the heatercircuit. Y A heating coil 27 -may be wound around the upper end of the thermostat to prevent condensation of mercuryon its inner surface. This coil may beiin series or' parallel with the main heating coil, or in La separate` circuit.

The upper and lower electrodes `of the crystal are connected across a resistance ,2S-in the grid circuit of a vacuum tube oscillator which is marked Oscillator andis of conventional design, theplateV circuit :providing an inductive reactance which causes theicrystal to oscillatev through energy fed back to the crystal through the inter-electrode capacity of the tube.

In a preferred embodimentof the invention the threads used aresilk, such as may be pur- Losetas chased in any dry-goods store. However', threads of othermaterial may be used, or thin pieces of woven fabric, or pieces of paper. In Fig. 2, reference numeral 31 designates a piece of fabric which is cushioned between the crystal 5 and the upper electrode 6, and numeral 32 designates a piece of fabric which is included between thecrystal 5 andthe lowerv electrode 4. Inlig. 3. numerals 33 and;'lqfdenatepeces of thin corrugated paper which are included between the crystal and the upper and lower electrodes, respectively. The fabric or paper may cover -the entire surfaces of the crystal or *only .a` portion thereof, and may be placed loosely inpposition 4,or laixed to the surfaces of lthe crystal orto the; adjacent electrode surfaces.

The theory ofthe operation of the invention has not been established. The necessary and suflicientcondition seems to be that some resilient spacing means be used between the crystal and its electrodes toovercome the tendency of the. crystal to` stick` to" the electrodes. If the threads are tied ,tightlyVv the results are less favorablethan if they are tied loosely.

What isclaimed is: Ll c i 1 f Y 1..A quartz crystal, electrodes therefor,- and resilient threads cushioned between said crystal and at least one of said electrodes.

2. A piezoelectrical crystal, electrodes therefor, and silk threads cushioned between said crystal and each of the said electrodes'.

3. A piezoelectric crystal, electrodes adjacent thereto and between which the crystal is freelyA positioned, and resilient fabric interposed betweensaid crystal and each of the said electrodes. U v

4. A quartz crystal, electrodes adjacent thereto between which'the crystal is-loosely positioned, and a layerl of .resilient material interposed between and. contacting said-crystal and each of the said electrodes.y v v l5. A piezoelectriccrystal holder comprising in combination a pair of electrodes, a piezoelectric` crystal interposedy betweenfsaid electrodes,-and a plurality of silkthreads attached to and surroundingsaid piezoelectric crystal and arranged tocontact atleast one electrode.

' 6..A piezoelectric `vcrystal, a-plurality of elec,- trodes, one of which supports-said crystal and another of which is supported only thereby, and thin material included betweensaid crystal and at least one ofthesaid electrodes. 1

7. A. quartzwcrystal, electrodes therefor, between which the crystal is freely' positioned, and resilient paper l vinterposed -between' said-V crystal and each of-theele'ctr'odes.A f d jhwILLrAM M. KNoTrpf 

